This invention relates in general to the use of radar techniques to detect minute body movements which are associated with cardiac and respiratory activity. The invention is based on the principle that breathing and heartbeat produce measurable phase changes in electromagnetic waves as they reflect off of a living person. The invention offers significant advantages over other similar and earlier approaches, including greater sensitivity, lower radiated power, improved reliability and lower cost.
Functionally, the non-invasive, electromagnetically-based Vital Signs Monitor (VSM) is an extremely sensitive motion detection system capable of detecting small body motions produced by respiratory and cardiac functioning. Motion detection is achieved by transmitting an interrogating electromagnetic field at the target of interest, and then measuring the time-delay of the return signal reflected back from the surface of the target. When the target surface is moving, as does the surface of the chest in conjunction with respiratory and cardiac activities, corresponding variations will be observed in the measured time delay. The observed variations can be used to determine motion-related target parameters such as displacement and velocity.
In the medical field, it is essential that a subject's respiration and heartbeat be capable of being measured. The medical profession is accustomed to voltage-derived electrocardiogram waveforms for monitoring heartbeat. Most respiration monitors also require physical connection to the subject's body. Many commercially-available devices are available for measuring heart and respiration rates, but most of them are electrode-based requiring physical contact with the subject. Devices requiring physical contact, however, are difficult to use on children susceptible to sudden infant death syndrome (SIDS) or burn patients who cannot tolerate the touch of electrodes. Many infants wear sensors while they sleep that trigger an alarm if their breathing stops, but electrodes attached to the child can be jarred loose as the infant tosses and turns.
The invention has similarities with motion-detection systems based on ultrasonic or optical techniques. However, an electromagnetically-based approach offers several advantages for monitoring of vital signs-related motions. For example, with proper antenna design, an interrogating electromagnetic field will suffer minimal attenuation while propagating in air (unlike ultrasonic signals which propagate poorly in air). Thus, the electromagnetically-based Vital Signs Monitor can easily be used in a completely non-contacting mode and can, in fact, be placed an appreciable distance from the test subject if required. Electromagnetic signals in the microwave band are also capable of penetrating through heavy clothing. This offers advantages over optical techniques which would have a difficult time of detecting motion through even thin clothing. Another feature of an electromagnetically-based approach is that the system could be designed to simultaneously interrogate the entire chest surface and provide information pertaining to any respiratory or cardiac function manifested as chest wall motions. Conversely, by modifying the antenna design, a localized region of the chest surface could be interrogated to obtain information about some specific aspect of respiratory or cardiac function. Such versatility would be difficult to achieve with other motion detection techniques.
In the prior art the patent to Allen, U.S. Pat. No. 4,085,740 discloses a method for measuring physiological parameters such as pulse rate and respiration without electrodes or other sensors being connected to the body. A beam of electromagnetic energy is directed at the region of interest which undergoes physical displacement representing variations in the parameter to be measured. The phase of the reflected energy when compared with the transmitted energy indicates the amount of actual physical movement of the body region concerned. The method does disclose simultaneous detection and processing of respiration and heart beat; however, frequency modulation is not used, therefore and the subject must be reasonably still. The receiver includes two channels and in one of them the received signal is mixed with a signal substantially in quadrature with the transmitted signal to maximize amplitude output in those cases in which the received signal is 180.degree. out of phase with the transmitted signal.
The patent to Kaplan, et al., U.S. Pat. No. 3,993,995 discloses an apparatus for monitoring the respiration of a patient without making physical contact. A portion of the patient's body is illuminated by a transmitted probe signal with the reflected echo signal detected by a monitor. The phase difference between the transmitted and reflected signals is determined in a quadrature mixer which generates outputs indicative of the sine and cosine of the difference signal. These two outputs are coupled to differentiators and when both time derivatives are substantially zero an x-ray unit is triggered since it represents an instant of respiration extrema (apnea). The outputs of the quadrature mixer are also coupled to a direction of motion detector which indicates inhalation or exhalation.
The patent to Kearns, U.S. Pat. No. 4,289,142 discloses a respiration monitor and x-ray triggering apparatus in which a carrier signal is injected into the patient's thorax which is indicative of the transthoracic impedance of the patient. This impedance changes as a function of the respiration cycle. The carrier signal is injected through electrodes coupled to the patient's thorax. The transthoracic impedance has an alternating current component having a respiratory component between 0.2 to 5 ohms and a cardiac component varying between 0.02 to 0.2 ohms.
The patent to Robertson et al., U.S. Pat. No. 3,524,058 discloses a respiration monitor which uses body electrodes to direct an electric current to a particular part of the patient's body where changes in electrical impedance provide output signals that vary with respiration.
The patent to Bloice, U.S. Pat. No. 3,796,208 discloses an apparatus for monitoring movements of a patient including a microwave scanner (doppler radar) which creates a movement sensitive field surrounding part of the patient. Movements of the patient create disturbances in the field which are monitored and which trigger alarm circuitry.
Also in the prior art, apexcardiograms (ACG), which represent a contact technique for measuring small chest surface motions overlying the cardiac apex, have been used to estimate cardiac contractility, left ventricular end-diastolic pressure, pressure changes during atrial systole and cardiac ejection fraction, in addition to diagnosing myocardial wall abnormalities and dysfunction. One of the problems associated with the use of an ACG for the estimation of cardiac function is that the motions recorded are indicative only of activity at the apex of the heart and not of the heart as a whole. Analysis of the VSM waveform is potentially a better choice for estimation of cardiac function since the larger beamwidth of the VSM antenna actually integrates motion over a certain area of the chest. In addition, since the VSM waveform appears to contain information related to aortic and other vascular pulses, it can be used to measure pulse transit times directly out of the heart into the aorta. This measurement can potentially be used as a non-invasive, non-contact means of estimating blood pressure as discussed by L. A. Geddes, M. Voelz, C. F. Babbs, J. D. Bourland and W. A. Tucker in "Pulse Transit Time as Indicator of Arterial Blood Pressure," Psychophysiology, Vol. 18, No. 1, pp. 71-74, 1981. This paper showed that the pulse-wave velocity in the dog aorta increased linearly with increasing diastolic pressure. Similarly, pulse pressures may be related to either the magnitude of the aortic peak in the VSM waveform, or possibly to the rate of rise of this peak.